The present invention relates to a multi-stage, post-mold temperature conditioning method and apparatus used in conjunction with an injection molding machine having an aggressive cycle time and a high output. Temperature conditioning means either cooling or cooling and re-heating or re-heating only.
Reduction of the injection molding cycle time is a major task when forming articles in huge volume. This is for example the case of PET preforms that are formed using high cavitation molds, such as for example the 72 or 96 cavity molds made by Husky Injection Molding Systems. One option to reduce the molding cycle time is to limit the residence time of the preforms in the mold closed position by shortening the cooling step by a few seconds and thus ejecting the preforms from the mold sooner. The temperature gradient across the walls of a preform indicates formation of inner and outer skin layers which are colder than the internal layer. This is caused by the fact that in the closed mold position, cooling is done from both the cavity and the core side.
Numerous attempts have been made in the past to improve the post-mold cooling process when forming PET preforms. U.S. Pat. No. 4,209,290 to Rees et al., for example, illustrates a system in which preforms or parisons to be blow-molded into bottles are produced in cavities of a lower mold half of a vertical injection-molding machine with the aid of respective cores depending into the cavities from an upper mold halve when the mold is closed. As the two mold halves separate, the parisons adhere to the respective cores for which they are subsequently discharged into respective nests of a transfer box or respective cells of a blow-molding unit interposed between the mold halves. In the first instance, the parisons are cooled by a circulating air stream while the transfer box is laterally withdrawn before the start of a new injection-molding cycle, the rigidified parisons are then released from their nests to drop into pockets of an underling conveyor transporting them to a blow-molding station. In the second instance, the cores are hollow and descend with their parisons into the blow-molding cells whose split walls are closed around the cores by fluidic or mechanical means. There the parisons are inflated by air injected through the cores whereupon the finished bottles are extracted from these cells after the blow-molding unit has been laterally withdrawn and while a new injection cycle takes place.
U.S. Pat. No. 4,836,767 to Schad et al. illustrates an apparatus for producing molded plastic articles which apparatus is capable of simultaneously producing and cooling the plastic articles. The apparatus has a stationary mold half having at least one cavity, at least two mating mold portions, each having at least one core element, mounted to a movable carrier plate which aligns a first one of the mating mold portions with the stationary mold half and positions a second of the mating mold portions in a cooling position, a device for cooling the molded plastic articles when in the cooling position, and a device for moving the carrier plate along a first axis so that the aligned mold portions abuts the stationary mold half and the second mating mold portion simultaneously brings each plastic article thereon into contact with the cooling device.
U.S. Pat. No. 5,232,715 to Fukai illustrates an apparatus for cooling and solidifying a preform. The apparatus involves introducing a preform in a heated stated released from an injection mold into a cooling tube having a bottom opening and an upper opening through which cooling air flows from the bottom opening to the upper opening. The bottom opening is provided with a fan for cooling air. The preform is forcibly cooled from the interior and the exterior by cooling air flowing in a turning state between the preform and a coolant supplied to a cooing core which has been inserted into the preform.
Japanese Patent Document No. 7-171888 to Hirowatari et al. relates to a forced cooling apparatus for preforms. The apparatus comprises a cooling tool for supporting a plurality of preforms, a first air cooling means disposed above the cooling tool for spouting cooling air inside of the preforms, and a second air cooling means disposed beneath the cooling tool for spouting cooling air outside of the preforms.
U.S. Pat. No. 5,772,951 to Coxhead et al., assigned to the assignee of the instant application and which is hereby incorporated by reference herein, relates to a preforms storage and temperature conditioning device that connects a single or several injection molding machines to a blow molding station. This allows the injection molding machine to run while the blow molding does not. The preforms storage device of the '951 patent receives a plurality of preform carriers, each holding molded preforms. According to the '951 patent, the preforms released from the mold are cold enough to avoid problems like crystallinity, deformation and surface damage. These preforms are stored and temperature conditioned so that they are ready, in terms of temperature and number, for the step of blow molding. The '951 patent does not teach a multi-stage and compact cooling station that receives still very warm and freshly molded preforms. The '951 patent also does not teach a method and a post mold cooling station used to prolong the cooling time of the preforms without affecting the injection cycle time.
Post-mold cooling becomes more critical and difficult to implement as the injection molding cycle time is reduced. This is especially true when forming blowable preforms made of resins such as PET. Because the in-mold cooling is shorter than usual, the preforms ejected from the mold are solidified outside and inside but are still very warm and hot and not fully solidified inside the walls. This internal heat may re-heat and reduce the strength of the inner and the outer skin layers of the preforms, and thus the preforms are vulnerable to damage or to sticking together. Currently, the preforms molded using a fast cycle time and that are post-mold cooled using known methods and equipment are still sufficiently warm when they are dropped onto a conveyor. They can thus stick together or be damaged by the impact to the conveyor or other preforms during manipulation. If the cooling time is prolonged on a take-out plate or other known cooling devices somehow connected to the injection molding machine, this will increase the cycle time.
It remains very important to develop a method and apparatus for maintaining the temperature of the skin layers as low as possible after opening the mold and during the ejection, post mold cooling and handling of the preforms.
After the injection process, the molded preforms are directed to a blow mold where they are blown into finish articles such as bottles or jars. Before the blowing process, the preforms are re-heated to the blowing temperature. The blowing process can take place immediately after the injection process using for example an integrated injection-blow molding machine. In some instances, the preforms blown using this approach are not completely cooled so less heat is needed during re-heating. In a different approach, the preforms are fully cooled after molding and then stored and sent to a different location for the full re-heating and final blow molding process.
The re-heating process require a careful monitoring so that all the preforms have the same temperature before the blowing step. This is not easy to achieve using a compact and simple equipment.
It remains important to provide a simple method and high output apparatus for cooling and re-heating the preforms after the injection and prior to the blow molding process.